Search warrant is a court order authorizing the examination of a place for the purpose of discovering evidence for presentation in criminal prosecutions.
To obtain a search warrant, an officer must personally appear before, or speak directly with, a judge or magistrate. The officer must present information that establishes probable cause to believe that a search would yield evidence related to a crime. Probable cause exists when an officer has either personal knowledge or trustworthy hearsay from an informant or witness. The officer must fill out an affidavit stating with particularity the person to be seized and searched, the area to be searched, and the objects sought. The warrant need not specify the manner in which the search will be executed
The judge won't issue the warrant unless she has been convinced that there is probable cause for the search -- that reliable evidence shows that it's more likely than not that a crime has occurred and that the items sought by the police are connected with it and will be found at the location named in the warrant.
Generally, a law enforcement agency planning to execute a search warrant will make preparations prior to entry to a premises. The officers involved in the search will attempt to gather information obtained from reliable sources, such as undercover cops or informants, as to the layout of the premises being searched and the location within the premises of the items for which the search is conducted. When there is a flight risk involved, officers will try to surround the premises, guarding all doors, windows, and other possible escape routes.
The Supreme Court has ruled that individuals in automobiles have a reduced expectation of privacy, however, there must be probable cause or a reasonable suspicion of criminal activity before a vehicle may be searched. Items in "plain view" may be seized; areas that could potentially hide weapons may also be searched. Any part of the vehicle can be searched if probable cause exists. However, passengers may not be searched without probable cause or consent from the passenger(s) to search their persons or effects.
In limited situations the police may search without a warrant, but they cannot use what they find at trial if the defense can show that there was no probable cause for the search.
